Explain Lua code

What this Lua sample shows

tables

The table is Lua's one data structure, and the sample uses it three ways: scores maps names to points, each { name = name, score = score } is a record, and rows and lines are lists numbered from 1. Reading a missing key gives nil, so self.scores[name] or 0 starts a new player at 0.

metatables

setmetatable({ scores = {} }, Leaderboard) makes a board that looks up any key it doesn't have in Leaderboard, because Leaderboard.__index is Leaderboard itself. The board holds only scores, so board:add finds add on Leaderboard. board:add("nova", 30) is short for board.add(board, "nova", 30), and that first argument is self.

Good to know

• With nova adding 30 and then 20, blitz 45 and echo 50, board:top(3) returns 1. echo: 50, 2. nova: 50 and 3. blitz: 45.
• pairs visits keys in no set order, so table.sort decides the order: higher scores first, and a tie goes by name, which is why echo comes before nova on 50 every time.
• Names are compared as text, and in this run capital letters sorted before lowercase ones: three players tied on 10 come out as Bolt, Zed, alpha. Compare a.name:lower() < b.name:lower() to ignore case.
• math.min(count, #rows) stops at the players there are: top(10) with three players returns three lines, and an empty board returns an empty table.
• The sample uses only standard Lua, no Roblox APIs, and was run with Lua 5.5. In Roblox it would usually sit in a ModuleScript that ends with return Leaderboard, which other scripts load with require.

Watch out: Methods need the colon. board.add("nova", 5) with a dot passes "nova" as self and 5 as the name, and fails with attempt to index a nil value (field 'scores'). Leaving out the points fails too, with attempt to perform arithmetic on a nil value (local 'points').